1. Field of the Invention
The present invention relates to a furnace for and method of drawing optical fibers and specifically to a graphite induction furnace for drawing optical fibers from large glass preforms and method for drawing optical fiber or smaller diameter preforms from such large-diameter preforms.
2. Description of the Related Art
Furnace designed for drawing optical fibers may be divided into two main categories, defined by the method of heating, which can be conductive heating or inductive heating. Inductive heating furnace are generally preferred, mainly because of their flexibility of use. Induction furnaces may then be further categorized as using heating elements made of graphite or of zirconia.
These furnaces generally comprise a cylindrical susceptor (either graphite or zirconia), defining the heating zone of the furnace, surrounded by insulating material which is in turn surrounded by an induction coil. When the coil is energized, an electromagnetic field is generated which couples to the susceptor (generally preheated) increasing the temperature thereof and generating the desired hot zone for melting the preform inserted into the cylindrical susceptor. Extension, in the form of cylindrical tubes, may be provided on the top (the so-called top-chimney) and/or on the bottom (the so-called bottom chimney) of the heating zone of the furnace.
Graphite furnaces use ultra-pure graphite heating elements. A typical problem with these furnaces is the oxidation of the heating element. The reactivity of the graphite heating element also pollutes the reaction area. Therefore, the life of the heating element is short and frequent replacement of the element is required. These problems are commonly overcome by providing an inert, protective atmosphere inside the furnace. Usually, argon or nitrogen is employed for the purpose. In U.S. Pat. No. 4,154,592 the use of helium as conditioning gas is disclosed. Its high thermal conductivity is helpful for better thermal stability and uniformity in the region close to the tip of the preform.
Several documents have discussed furnaces for drawing optical fiber. For example, IT 1,077,118 describes a high-frequency electric furnace comprising a tubular graphite core enclosing the items to be heated, and in turn enclosed by an induction winding to which high-frequency current is supplied. A housing enclosing the core has a wall composed of sections placed one on top of the other, and inert gas is delivered to the inside of it. The inert gas surrounding the core prevents oxidation of the core while the furnace is in operation. The gas is introduced at the center of the furnace and is allowed to escape between the outer sections. The furnace is capable of operating at temperatures of about 2000° C.
U.S. Pat. No. 4,174,462 describes a large-diameter, general-purpose, graphite induction furnace. No conditioning gas is used in the furnace. Instead, the furnace is kept full of material to avoid oxidation damage to the graphite susceptor. The susceptor is insulated with powdered carbon black. The carbon black is contained within an asbestos and concrete cylinder.
DE 3,025,680 describes a graphite induction furnace for drawing optical fiber. The heating element is coated with a 10-micron-thick protective layer that does not react with glass, even at high temperatures, e.g., platinum or iridium. The coating prevents any material transport between the heating element and the glass.
U.S. Pat. No. 4,154,592 describes a method of forming optical fiber by disposing a preform in a cylindrical muffle. The muffle is heated with a resistance heating element to a temperature sufficient to cause a first end of the blank to reach the drawing temperature of the preform. Fiber is drawn from the first end of the preform while a helium-containing gas flows through the muffle in such a direction that it is exhausted from that end of the muffle from which the filament is being drawn.
U.S. Pat. No. 4,400,190 describes a graphite resistance furnace for drawing optical fiber. The graphite heating element is generally cylindrical with a centrally located heating chamber of reduced cross-sectional area and areas of enlarged diameter on each end of the heating chamber. Inlet and exhaust tubes are inserted into the areas of enlarged diameter. The internal diameter of the centrally located heating chamber and the inner diameters of the inlet and exhaust tubes are substantially equal. The transitions from the centrally located heating chamber to the areas of enlarged diameter within the heating element may be tapered.
One problem associated with open-ended furnaces through which conditioning gas flow, however, is that turbulence around the preform and the newly drawn fiber can cause diameter variations and consequently affect fiber optical performances. One possible cause for such turbulence is the updraft of ambient air, as disclosed in U.S. Pat. No. 5,284,499 and Patent EP 0 329 898, which suggest preventing updraft by inserting a shutter at the bottom of the furnace.
Another possible cause of turbulence is related to differences in the cross-sectional area of the conditioning gas flow around the preform. U.S. Pat. No. 4,400,180 suggests to minimize this source of turbulence by properly adjusting the ratio between the diameter of the heating element and the diameter of an extension provided to the heating element.
U.S. Pat. No. 5,545,246 suggests to introduce an additional flow of conditioning gas in the draw down zone of the preform, in order to reduce turbulence which may be caused in the conditioning gas flowing along the preform in this zone.
In the article “Combination furnace for drawing large optical fiber preforms at high speed”, M. Rajala et al., International Wire & Cables symposium 1998, a graphite induction furnace is disclosed wherein the susceptor is a graphite element having a 80 mm inner diameter.
The other type of furnaces, i.e. induction-heated zirconia furnaces, are of relatively simple construction and their ability to operate in air is a major advantage. Generally, the heating element in induction furnaces is an inert cylinder of ZrO2 stabilized with Y2O3.
One problem connected with the use of zirconia induction furnaces is the relatively high thermal inertia of zirconia. This results in quite long time intervals between two subsequent drawing operation, as the zirconia susceptor has to be naturally cooled before inserting the new optical preform. As a matter of fact thermal shocks caused by rapid changes in the temperature of the heating element constitute a serious problem for induction heated zirconia furnaces, thus preventing a forced cooling of it in between two subsequent drawing operations. In addition, when, for example, a furnace is shutdown due to a power failure or power supply problem, the zirconia cools through its structural transition, cracks, and must be replaced. This causes the furnace to be out of operation for a large amount of time, because the zirconia susceptor must be heated and stabilized before drawing can be initiated.
U.S. Pat. No. 4,450,333 describes a zirconia induction furnace for drawing fiber from a preform. The furnace has a centrally located tubular susceptor with a thin coating of the preform material (e.g., silica) on at least a portion of its inside surface. The thin coating on the susceptor prevents contaminating particles from migrating from small cracks in the inside surface of the susceptor onto the preform. A cylinder is positioned in concentric, spaced relation about the susceptor and is surrounded by an insulating grain. The cylinder prevents small particles emanating from the insulating grain from being drawn through larger cracks in the susceptor and onto the preform and/or the fiber.
U.S. Pat. No. 4,608,473 describes a zirconia induction furnace for drawing fiber from a silica preform. The furnace includes an axially located tubular zirconium dioxide susceptor. Prior to use, at least a portion of the inside surface of the susceptor is coated with a vapor-deposited silica “soot.” The silica soot is then consolidated at an elevated temperature. Such a technique substantially eliminates migration of zirconium dioxide particles from the susceptor to the preform and/or the fiber.
U.S. Pat. No. 5,284,499 describes a zirconia induction furnace for drawing optical fiber. Gases introduced at the top of the furnace form a boundary layer adjacent to the fiber, which passes through the furnace, along with the fiber, into a tube. The tube isolates the fiber from the ambient atmosphere so that the boundary layer of gases established in the furnace remains substantially uniform until the viscosity of the cladding layer of the fiber is high enough to minimize differential stresses around the circumference of the fiber. A planar shutter is positioned at the bottom of the tube to prevent the ambient atmosphere from entering the bottom of the tube.
EP 849 232 describes a zirconia induction furnace for drawing optical fiber. Separate gas supplies inject a conditioning gas into the furnace between the preform and the furnace wall. A first gas supply is provided at the entry end of the furnace space and a second gas supply is provided in the drawing taper region at the exit end.
Other examples of furnaces for drawing optical fiber include: FR 2,368,677, describing a graphite induction furnace having multiple bores through the susceptor; EP 329898 describing an induction furnace with a disk-like shutter closing the bottom end; GB 1,521,231 describing a graphite induction furnace with an inner sleeve of zirconia; GB 1,575,299 describing a graphite induction furnace with a coated susceptor; U.S. Pat. No. 4,142,063 describing a zirconia induction furnace; and U.S. Pat. No. 5,017,209 describing a furnace with a cylindrical heater surrounded by an anisotropic cylindrical insulation.
Commercial furnaces either of the graphite or of the zirconia type, suitable for optical fiber drawing are available, for instance, from the following manufacturers: Astro (USA), Centorr (USA), Heathway (UK), Lepel (USA), Stanelco (UK).
Conventional preforms which may be employed in commercial furnace are typically of about 40-50 mm diameter (when produced according to MCVD—Modified Chemical Vapor Deposition—technology) and up to 70-80 mm (for those produced according to the OVD—Outside Vapor Deposition—or VAD—Vapor Axial Deposition—technology). Based on the preform diameter, conventional susceptor diameters are typically about 70 mm (MCVD preforms) or about 100 mm (OVD or VAD preforms).
The increasing demand of optical fibers requires improvements in optical fiber production process, which improvements may relate to more than one aspect of the process, such as for instance, increase in the drawing speed of the fiber or reduction of the dead-time between one drawing and the subsequent one.
For instance, use of preforms with a diameter larger than the conventional (e.g. 100 mm or more) could increase the productivity of optical fibres, as it would allow to produce longer lengths of optical fiber using a single preform. Typically, the time required between the end of the drawing of a preform and the beginning of the subsequent one is of about 1-2 hours, depending on dimensions and materials of the furnace. By reducing the frequency of interruptions in the drawing process, an increase in optical fiber productivity can be obtained.